Kura runner-cache fallback to public cache when private node is unready

When a private Kura runner node becomes unready, cache dispatches now fall back to the public cache instead of keeping builds routed to the dead endpoint. Previously, a wedged Kura node-port server could cause module-cache downloads to time out because the runner-cache endpoint was a hard override. Now the server tracks a readiness heartbeat for private node-port Kura servers, and once that heartbeat is stale it stops handing out the private endpoint, letting builds fall back to the public cache and keeping CI from stalling.